Joint attention is transformed repeatedly as young children acquire new ways to coordinate actions and represent events. This important developmental process is guided by caregivers who bring to interactions symbol systems and the ability and willingness to complement a child[unreadable]s actions. Difficulties negotiating shared engagement with caregivers may deprive a child full access to a context that is crucial for essential accomplishments, including the acquisition of language. The proposed studies in this revised competitive renewal application continue a productive research program that is charting variations in the development of joint attention after infancy. Our focus for the next cycle is on how parents support joint engagement as toddlers become increasingly competent symbol users and how this process is impacted by developmental disorders, including autism spectrum disorders, Down syndrome, and severe speech delay, which are characterized by deficits in joint attention skills and/or problems acquiring symbols and using language. Three aims will be pursued. First, using an experimental study of the way mothers of typically-developing children, children with autism, and children with Down syndrome introduce novel words will be conducted to clarify how mothers create [unreadable]hot spots[unreadable] for language learning during periods of joint engagement. Second, new rating scales will be formulated for our standard observational procedure, the Communication Play Protocol, and used to investigate how the child[unreadable]s diagnostic group and language level affects the relation between the child and parent actions and the topics of their joint engagement. Third, two longitudinal clinical research studies will be performed to characterize more fully the development of joint attention in young children with developmental disorders and to discern how observations of how parents support a child[unreadable]s joint engagement may add to the prediction of diagnostic and language outcome. The first study observes 40 children with an autism spectrum disorder (ASD) and 40 children with a developmental delay at the time of initial diagnosis (18 to 24 months) and at 30 and 36 months in order to compare joint attention as it is rated during parent-child interactions, scored during standardized assessments with examiners, and reported by parents when they are asked about their child[unreadable]s behavior. Analyses will discern if ratings derived from observing children with parents, including those of parental support of symbol-infused joint engagement, will add to our capacity to predict diagnostic and linguistic outcome at 42 months of age. The second study is designed to explicate our recent finding that children with severe speech delay who participated in a parent-implemented augmented language intervention project significantly increased symbol-infused joint engagement from pre- (30 months) to post- (36 months) intervention. Parent actions during social interactions will be described to determine if they too changed during this time interval and whether changes in parent actions predict language outcome at 42 months of age.